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Lubricant-infused micro/nano-structured surfaces with tunable dynamicomniphobicity at high temperaturesDaniel Daniel, Max N. Mankin, Rebecca A. Belisle, Tak-Sing Wong, and Joanna Aizenberg Citation: Appl. Phys. Lett. 102, 231603 (2013); doi: 10.1063/1.4810907 View online: http://dx.doi.org/10.1063/1.4810907 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i23 Published by the American Institute of Physics. Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors
Lubricant-infused micro/nano-structured surfaces with tunable dynamicomniphobicity at high temperatures
Daniel Daniel,1,a) Max N. Mankin,2,a) Rebecca A. Belisle,3 Tak-Sing Wong,1,3,b),c)
and Joanna Aizenberg1,2,3,c)
1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA2Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA3Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge,Massachusetts 02138, USA
(Received 2 April 2013; accepted 23 May 2013; published online 12 June 2013)
Omniphobic surfaces that can repel fluids at temperatures higher than 100 �C are rare. Most state-
of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is
maintained throughout micro/nanotextures leading to high mobility of liquids. However, such
behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases
significantly. Here, we demonstrate a class of lubricant-infused structured surfaces that can
maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at
least 200 �C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of
1000. VC 2013 Author(s). All article content, except where otherwise noted, is licensed under aCreative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4810907]
The ability of surfaces to repel various liquids (i.e.,
omniphobicity) at elevated temperatures has broad practical
applications in refinery processes, ethanol concentration, fuel
transportation, district heating, and protective fabrics.1,2 State-
of-the-art approaches have been based on the “lotus” effect,
where micro/nano-structures are carefully designed to main-
tain an air layer between the structures, forming a stable inter-
face between the substrate and the applied liquid.3,4 This
superhydrophobic (SH) interface, however, becomes unstable
for low-surface-tension liquids due to their enhanced ability to
wet surfaces.5 Since surface tension decreases with increasing
temperature, which further destabilizes the liquid-air interface,
it is very challenging to design surfaces that can repel a wide
range of liquids at high temperatures.
Early work on omniphobicity employed photolithography
to create complex re-entry geometries on a surface that could
then repel liquids with surface tensions down to �17 mN/m
(e.g. heptane) at ambient temperature.6,7 Other approaches to
generating re-entry geometries that achieve omniphobicity
were later demonstrated.8–10 It was also shown that small liq-
uid droplets can bounce off an extremely hot superhydropho-
bic surface reminiscent of the Leidenfrost effect, provided that
the heat transfer between the solid and liquid is sufficient to
generate a vapor layer, which can only occur when the tem-
perature of the solid substrate Tsolid is significantly above the
liquid’s boiling point (Tsolid> 150 �C for water).11 Recent pro-
gress in creating superhydrophobic/superoleophobic materials
based on a fluorinated silica network and rare-earth oxide
ceramics addressed the problem of developing mechanically
robust surfaces that retain liquid-repellency (quantified by
contact angle at ambient temperature), after annealing/sinter-
ing of the substrate at high temperatures up to 1000 �C.12–14
While these surfaces have high-temperature stability during
their respective manufacturing processes and remain liquid-
repellent after cooling down to room temperature, they did not
demonstrate liquid repellency at elevated temperatures.
Oleophobic surfaces based on polymethylsilsesquioxane coat-
ing, on the other hand, have been shown to repel organic
liquids at 250 �C.15 Nonetheless, omniphobic surfaces that can
repel various fluids at elevated temperatures (when either
Tliquid or Tsolid or both are above 200 �C) remain rare due to
the fact that surface tensions of some organic liquids, such
as heptane and octane, can fall below 10 mN/m at
T> 200 �C.16,17
Recently, we developed a radically different concept in
designing omniphobic surfaces, termed as Slippery Liquid-
Infused Porous Surfaces (SLIPS), in which a suitably treated
micro/nano-structured surface is infused with a lubricant, as
shown in Fig. 1(a). Any foreign liquid droplet immiscible
with the underlying lubricant can then easily slide off at a
small tilting angle of <5�.18 We demonstrated how SLIPS
coating can be applied to a variety of solid substrates, includ-
ing polymers, metals, glass, and ceramics, such that arbitrary
materials begin to show superior liquid-repellency, anti-bio-
fouling, and anti-icing properties.19–21 Lafuma and Qu�er�eoutlined the requirements for thermodynamic stability for
such a system, which was further elaborated by Smith
et al.22,23 Here, we report that SLIPS retain their excellent
liquid-repelling properties at high temperatures up to
Tsolid,liquid¼ 200 �C for a wide range of liquids, including
those with low surface tension. This is in contrast with repre-
sentative SH surfaces that were shown to lose their liquid-
repellency even at moderate Tliquid� 90 �C.1
SLIPS can be created from a SH-surface by infusing it with
a lubricant such as perfluorinated oil. A micro-structured sur-
face consisting of a hexagonal array of posts with dimensions:
diameter d¼ 1 lm, height h¼ 10 lm, and pitch l¼ 4 lm, as
illustrated in the inset of Fig. 1(b), was fabricated using replica-
tion techniques described elsewhere.24 The final substrate was
a)D. Daniel and M. N. Mankin contributed equally to this work.b)Present address: Department of Mechanical and Nuclear Engineering, The
Pennsylvania State University, University Park, Pennsylvania 16802, USA.c)Authors to whom correspondence should be addressed. Electronic
addresses: [email protected] and [email protected]
0003-6951/2013/102(23)/231603/4 VC Author(s) 2013102, 231603-1
APPLIED PHYSICS LETTERS 102, 231603 (2013)
made of UV-cured polyurethane (NOA 61, Norland) that was
then fluorinated to render its surfaces SH. At room temperature,
this SH-surface repels water with a contact angle of 165�6 5�,but when 200 ml of hot water (Tliquid¼ 95 �C, stained with
methylene blue) was splashed onto the surface, the surface was
clearly wetted, as shown in Fig. 1(b). The same solid substrate
infused with perfluorinated polyethers (Krytox 100, DuPont,
lubricant thickness¼ 13 lm) retained its water-repellency even
at 95 �C.
To confirm that the decrease in surface tension, c, was
the cause of the transition from a non-wetting to a wetting
state, we measured the contact angle hysteresis (CAH) for
water-ethanol mixtures of increasing ethanol (EtOH) concen-
tration (0–70% wt.) on the SH-surface and SLIPS to simulate
the effect of decreasing water tension with increasing tempe-
rature, Tliquid, as summarized in Fig. 1(c). Near boiling, water
has a surface tension of 58 mN/m as opposed to its
room temperature value of 72.8 mN/m.25 For SLIPS, CAH
remained small (�2�) for all EtOH concentrations, whereas
for the SH-surface, there was a transition from non-wetting to
wetting when the liquid droplet reached a critical surface ten-
sion, 50< ccritical< 60 mN/m. The CAH for 6% EtOH was
18�, whereas for 7% EtOH, the receding contact angle was
�0�, while the advancing contact angle was 130�, i.e.,
CAH¼ 130�. This sharp pinning transition is consistent
with the wetting observed for water at Tliquid¼ 95 �C,
where cwater¼ 59.8 mN/m approaches c6%EtOH, shown as a
dashed line in Fig. 1(c). A similar wetting temperature,
Twetting¼ 94�6 1�, was observed for a SH-surface dipped in
hot water for 1 min to ensure that the surface and water were
at the same temperature, i.e., Tsolid¼ Tliquid.
While these Norland-polymer infused surfaces show
promise in applications that involve high-temperature water
transport and district heating, they are unsuitable for ultra-
high temperature applications involving liquids with high
boiling point, since Norland 61 polymer begins to degrade at
Tsolid� 150 �C. To investigate the effects of T> 100 �C, a
Teflon membrane (pore size �0.2 lm, thickness �50 lm),
which resists degradation up to 350 �C, was chosen as the
solid substrate. Fig. 2 demonstrates that a Teflon membrane
infused with perfluorinated Krytox 105 easily repels a 50 ll
droplet of crude oil at TSLIPS¼ 200 �C. Even at a small incline
of 10�, the droplet moved at a speed of �1.5 cm/s, whereas
the crude oil droplet on the bare Teflon membrane (SH state)
was pinned to the surface. The crude oil used contained
mostly light oil fractions with a boiling temperature of around
150 �C. SLIPS remained liquid-repellent even when the crude
oil droplet started to boil. At T¼ 200 �C, crude oil is known
to have a surface tension of<20 mN/m.26,27
It is important to note that liquid-repellency of the material
is associated with and can be described in terms of how quickly
liquid droplets can move on and hence be removed from the
surface. This in turn is significantly affected by the viscosity of
the lubricant, glubricant, since the viscous force associated with
the trapped lubricating film scales linearly with glubricant.23,28,29
Unlike for the case of SH-surfaces, liquid droplets on SLIPS
will, therefore, become more mobile with initial increase in
temperature, as glubricant decreases with increasing temperature.
We measured the velocity of 20 ll droplets of crude oil moving
on a heated Teflon membrane infused with various lubricants
(perfluorinated polyethers, DuPont Krytox 100, 103, and 105
with chain length n¼ 10–60 that have similar surface tensions
of c¼ 17 6 1 mN/m but different viscosities g101,103,105¼ 20,
140, and 800 cP at room temperature respectively) and posi-
tioned at small 5� tilt angle at temperatures TSLIPS ¼20–200 �C. The results are summarized in Fig. 3. Each
FIG. 1. (a) Schematic of superhydrophobic (SH-) surface (left) and SLIPS (right): For a SH-surface, there is a transition from non-wetting to wetting with increasing
T. SLIPS, on the other hand, remain liquid-repellent at high T. (b) Hot water (T¼ 95 �C) stained with methylene blue effectively wets SH-surface, while the same
surface infused with Krytox 100 remains dry and stain-free. (c) Plot of CAH on SH-surface (black squares) and SLIPS (red circles) as a function of surface tension
for water and water-EtOH solutions of increasing EtOh concentrations (2, 5, 6, 7, 10, and 20% wt.) (enhanced online) [URL: http://dx.doi.org/10.1063/1.4810907.1].
FIG. 2. Images at time t¼ 0–1.4 s of two 50 ll crude oil droplets on 10�
incline placed next to each other on a hot plate, one pinned to a Teflon
membrane and the other sliding at a speed of 1.5 cm/s on the same mem-
brane infused with Krytox 105. At t¼ 1.4 s, crude oil started to boil. Inset:
SEM of the Teflon membrane (scale bar 1 lm) (enhanced online) [URL:
http://dx.doi.org/10.1063/1.4810907.2].
231603-2 Daniel et al. Appl. Phys. Lett. 102, 231603 (2013)
measurement was repeated 3–5 times and a thermocouple was
used to monitor the temperature of the surface. For each case,
the velocity of the droplet, V, first increases with T, as shown in
Fig. 3(a), because the viscosity of Krytox oils decreases with
increasing TSLIPS, illustrated in Fig. 3(b), but only up to a criti-
cal temperature, Tc. With further heating beyond Tc, the Krytox
oil evaporation rate becomes significant and the velocity of the
droplets decreases due to pinning at exposed defects on SLIPS.
Tc was observed to increase with increasing oil index number,
because higher-index Krytox oils contain longer-chain poly-
ethers. Fig. 3(c) shows the results of thermogravimetric analy-
sis (TGA) for the different Krytox oils, which confirm that the
higher-index Krytox oils evaporate at higher TSLIPS, with maxi-
mum rate of evaporation occurring at Tmax¼ 170, 270, and
395 �C for Krytox 100, 103, and 105, respectively. Inferring Tc
directly from the thermogravimetric data is not trivial and it
was found that Tc occurs well below Tmax. Nevertheless, the
values of Tc for the different Krytox oils obtained,
Tc,100� 70 �C, Tc,103� 100 �C, Tc,105> 150 �C are consistent
with the maximum operating temperature T, shown by dashed
lines in Fig. 3(a), provided by the manufacturer.
To further demonstrate the effects of glubricant on the mo-
bility of liquid droplets, we measured the velocity of 20 ll
water droplets down an incline of 50� for Teflon membranes
infused with perfluorinated liquids (perfluoro-octane, FC-770,
Krytox 100–107) of increasing viscosity, glubricant¼ 1–2500
cP. Fig. 4(a) shows a time-lapsed image of such an experiment,
in which the positions of differently colored water droplets at
equally spaced time intervals are juxtaposed. Each 30 cm lane
was infused with different lubricants (increasing glubricant
from left to right); the time-lag, Dt, from start to finish was
0.4 s. Fig. 4(b) shows a log-log plot of the velocity of water
droplets and the viscosity of lubricant. For the most viscous
lubricant, Krytox 107, which is �2500 times more viscous
than water, the droplet moved at a speed of about 1 mm/s,
whereas for perfluoro-octane, which is as viscous as water
(glubricant� 1cP), the maximum velocity at the end of the
30 cm track reached an extremely high value of �0.5 m/s. To
put this into perspective, a similarly sized water droplet moves
down a vertically inclined glass at a speed of �1 cm/s. In Fig.
4(c), the plot of the droplet displacement on perfluoro-octane-
infused surface with time shows a quadratic relationship, i.e.,
the droplet was accelerating at a constant a¼ 1.15 m/s2, c.f. gsin(50�)¼ 7.5 m/s2. The discrepancy between the gravitational
acceleration g sin(50�), and the observed acceleration, a, is
probably due to friction between the underlying solid substrate
and the liquid droplet. For Teflon infused with other lubricants,
the droplets achieved their terminal velocities.
The results of the experiments depicted in Figs. 3 and 4
demonstrate that it is possible to tune the velocity of droplets
on SLIPS continuously by a factor of 1000 on the same solid
substrate by either (1) changing the temperature of the surface
or (2) using lubricants of different viscosities. This capability
has broader implications in terms of droplet manipulation on
surfaces. On a textured, SH-surface, droplet mobility is dic-
tated by the static contact angle hs and CAH, but the two
FIG. 3. Characterization of lubricant viscosity and stability at high temperatures and the resulting velocity of the moving liquid droplets. (a) Velocities of 20 ll
droplets of crude oil moving down a Teflon membrane infused with perfluorinated Krytox oils 100, 103, and 105 (K-100, 103, and 105) inclined at 5� for tempera-
tures T¼ 20–200 �C. Vertical dashed lines denote the maximum operating temperature for the different Krytox oils. (b) Viscosities of the corresponding Krytox
oils, g100, g103, and g105 as a function of increasing T. For a fixed T, g105� 10 g103� 100 g100. (c) Thermogravimetric analysis of Krytox oils demonstrates that the
rate of evaporation peaks at increasing T for higher index number. Solid lines represent fractional mass left, while dashed lines represent the rate of fractional
mass loss.
FIG. 4. (a) Time-lapsed image (total time-lag Dt¼ 0.4 s) of 20 ll water drop-
lets down a 50� incline for Teflon membrane infused with lubricants with
increasing viscosity: (left to right) perfluoro-octane, FC-770, Krytox oils
100, 103, 105, and 107. (b) Log-log plot showing dependence of droplet ve-
locity on lubricant viscosity. (c) Distance traveled by the water droplet with
time on the perfluoro-octane-infused surface. Water did not reach terminal
velocity and instead accelerated at constant a¼ 1.15 m/s2 (enhanced online)
[URL: http://dx.doi.org/10.1063/1.4810907.3].
231603-3 Daniel et al. Appl. Phys. Lett. 102, 231603 (2013)
parameters do not sufficiently characterize droplet behavior
on SLIPS. The water droplets on different lubricant-infused
SLIPS have similar hs¼ 115�6 2� and CAH� 3�, but never-
theless travel with a wide range of velocities, showing the im-
portance of the lubricant as an additional source of friction/
resistance for the droplet movement. Our results also suggest
that within different temperature regimes, there is an opti-
mum lubricant that maximizes the velocity of droplet: Krytox
100 for TSLIPS< 70 �C, Krytox 103 for TSLIPS¼ 70–120 �C,
and Krytox 105 for TSLIPS¼ 120–200 �C. We did not measure
the mobility of crude oil droplets for TSLIPS> 200 �C for rea-
sons of safety, but TGA indicates that when infused with
Krytox 105, SLIPS can retain its liquid-repellency up to
TSLIPS� 300 �C, as depicted in Fig. 3(c). The lifetime of
SLIPS at high temperatures is closely related to the evapora-
tion rate of the lubricant and therefore increases with increas-
ing Krytox index. (See supplementary material.32)
Finally, SLIPS exhibit a much better heat transfer rate
than SH-surface, which may be relevant in some industrial
processes where heat transfer is important.30,31 20 ll water
droplets were placed on a SH-surface and its SLIPS counter-
part (same surfaces as Fig. 1) that were kept at a temperature
Tsolid¼ 85�6 5� using a thermal plate. The temperatures of
the water droplet and the surfaces were measured using an
infra-red camera, as shown in Fig. 5. Initially, both water
droplets were at room temperature, but at equilibrium, the
cores of the water droplets were at different temperatures:
Tcore,SLIPS¼ 45�6 2�> Tcore,SH¼ 32�6 2�. The equilibrium
temperatures, Tcore,SLIPS and Tcore,SH remained constant until
all of the liquid droplets have completely evaporated.
Potentially, this heat transfer rate can be further controlled
by adjusting the thermal conductivity of the lubricants.
To summarize, we have demonstrated a system that is
capable of repelling a wide range of liquids, from simple
newtonian fluids such as water to a low-surface-tension com-
plex fluids such as crude oil, across a wide range of tempera-
tures up to 200 �C. In comparison, most state-of-the-art
SH-surfaces fail at moderate T< 90 �C. We also outlined
how to manipulate droplet’s mobility on SLIPS continuously
on the same solid surface by a factor of 1000 by either
changing the temperature or using different lubricants. We
believe that such tunable, robust omniphobic surfaces will
have many uses in industrial processes that involve liquid
manipulation and transport in challenging, high temperature
conditions.
We thank P. Kim for helpful discussions regarding
TGA. M.N.M. gratefully acknowledges a Fannie and John
Hertz Foundation Graduate Fellowship and a NSF Graduate
Research Fellowship. The work was supported partially by
the ONR MURI Award No. N00014-12-1-0875 and by the
Advanced Research Projects Agency-Energy (ARPA-E),
U.S. Department of Energy, under Award Number DE-
AR0000326. We acknowledge the use of the facilities at the
Harvard Center for Nanoscale Systems supported by the
NSF under Award No. ECS-0335765.
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details about the effect of using different lubricants on SLIPS lifetime.
FIG. 5. Infra-red image of a 20ll water droplet on a superhydrophobic
surface (left) and SLIPS (right) taken normal to the surface. Inset: SEM of the
solid substrate with a hexagonal array of posts (diameter d¼ 1 lm, height
h¼ 10lm, and pitch l¼ 4 lm) used in the experiment. Scale bar 5 lm.
231603-4 Daniel et al. Appl. Phys. Lett. 102, 231603 (2013)